Add shortened links to install commands

[theletterf]

This adds shortened ela.st script to install commands. It also makes the first curl command silent.

[jamietanna]

🛑 If I'm honest, I'm rather concerned about this being a recommendation:

• generally as a curl | sh
• then also throwing in sudo here
• the fact that this is a short URL that could be modified to point to any random location
• not teaching Elasticians to audit the scripts they're executing with superuser privileges

Looking up this pattern generally results in many articles such as https://lukespademan.com/blog/the-dangers-of-curlbash/ and https://security.stackexchange.com/questions/213401/is-curl-something-sudo-bash-a-reasonably-safe-installation-method and https://macarthur.me/posts/curl-to-bash/

[theletterf]

I understand the concern (and thanks for the suggestion, I've already applied it). It's a popular debate. However:

• We need a convenient, fast way of installing and updating docs-builder until we've packages set up.
• We control the ela.st links. Should Elasticians not trust Elastic links? Will the same happens with packages?
• We're not asking folks to run these instructions for CI/CD.

See https://gnu.moe/wallofshame.md for counterarguments. Even Rust uses this method for local installs.

[theletterf]

I've removed sudo from the command, as the user will be prompted interactively anyway.

[theletterf]

This makes the script a little more robust and makes the sentence about manual downloads more visible. #1255